Transcription
what is it?
The mechanism by which one strand of the DNA molecule is used as a template to yield an RNA molecule
The gene is the region of the DNA that is transcribed
The RNA product may be mRNA, tRNA, snRNA, or another type of RNA
Gene: A hereditary unit occupying a specific position within the genome
The information content of genes is in the form of specific sequences of nucleotides along strands of DNA.
Similarities between DNA replication
Fundamentally similar chemical mechanism
Its polarity ( Direction of synthesis) addition of nucleotides to the -3 OH
Use of a template
Stages of initiation, elongation and termination
RNA
Generally one stranded
Ribose as the sugar
Bases used = U,C,A,G
DNA
Deoxyribose as the sugar
Double stranded
Bases used: T,C,A,G
FORMATION OF A PHOSPHODIESTER BOND = CONDENSATION REACTION
Difference between DNA replication
Transcription only use RNA polymerase, so does not require a primer
Generally involves a limited section of the DNA
Within a transcribed segment only one DNA strand serves as a template
Basic structure of a gene
The promoter = bases pair sequence that specifies where transcription begins
The coding sequence is a base - pair sequence. It is where the coding information a polypeptide chain is specified, if this is a gene for mRNA
The terminator is a sequence that specifies the end of RNA transcript
Flanking regions
starts at 5' ends at 3'
Leader length is variable
3' trailer aids stability, long sequences of Us cause instability
Template strand: Is what is used as a template strand in the synthesis of mRNA
Coding strand: is not used as a template, but is identical in sequence to mRNA / DNA U's are T's
Promoter sequences: A stretch of DNA sequence that lies upstream that is required to direct the inititation of transcription by an RNA Polymerase
RNA polymerase
bacteria have a single type of RNA polymerase that catalyses synthesis of mRNA, rRNAs and tRNAs encoded in the bacterial chromosome
Synthesis of RNA Transcript
Initiation
Elongation
Termination
RNA polymerase 2 Binding and initiation of transcription
Transcription factors mediate the binding of RNA polymerase and the initiation of transcription
The completed assembly of transcription factors and RNA polymerase 2 bound to a promoter is called a transcription initiation comple
Promoter called a TATA box is crucial in forming the initiation complex
RNA polyermase moves along the DNA, it untwists the double helix 10-20 bases at a time
Requires for the RNA polymerase enzyme to be released and also the RNA product to be released
Prokaryote termination
Rho dependent: Hexameric protein
interaction activates an ATPase of Rho that is associated with its movement along the RNA in the 3' direction until it unwinds RNA-DNA hybrid at the active site of RNA polymerase
Termination in Eukaryotes
RNA pol 1 genes trancription is stoppped using a termination factor
RNA pol 111 genes ends after transcribing a termination sequence
RNA pol 11: Connected mechanism of transcription termination and RNA 3' end processing,
Cells modify RNA after transcription
Enzymes in nucleus modify pre-mRNA before the genetic messages are dispatched to the cytoplasm
during processing both ends of the primary transcript are usually altered
Alteration of mRNA ends
3' end get a poly A tail
5' end gets a nucleotide 5'cap
Functions: Protect mRNA from hydrolytic enzymes - They help ribosomes attach to the 5'end - They seem to facilitate the export of mRNA
mRNA processing the 5'
7- methyl guanosine is added by a triphosphodiester bond - It provides binding for ribosome and also protects the 5' end
Introns: Long noncoding regions of nucleotides
Exons: are eventually expressed and translated into amino acid sequences
RNA Splicing removes introns and joins exons, creating a continuous coding sequence
Spliceosomes: Consist of a variety of proteins and several small nuclear ribonucleoproteins that recognise and splice sites